In 3GPP (Third Generation Partnership Project), the work of standardizing the fourth-generation mobile communications system is currently under way. “LTE (Long Term Evolution)” being one of data communications specifications developed by 3GPP is a long-term advanced system aiming IMT-Advanced that is the fourth-generation (4G), and is also called “3.9G (Super 3G).”
The radio communication system of LTE includes a terminal station (UE: User Equipment) and a base station (eNodeB: evolved Node B) of LTE. Communication from the UE to the eNodeB is called uplink. Communication from the eNodeB to the UE is called downlink.
In addition to a normal terminal that is used directly by a person, such as a conventional mobile phone or PC (Personal Computer), there is also MTC (Machine Type Communication) such as a meter, vending machine, or electronic advertisement as the UE other than the one used directly by a person.
The work of developing the standard of Release 11 related to LTE of 3GPP dealt with CoMP (Coordinated Multi Point Transmission/Reception: coordinated multi point transmission/reception) as a key technology to improve the throughput of a UE located at a cell edge. CoMP is a technology in which a plurality of eNodeBs transmits and receives data simultaneously to and from one UE to promote the reduction of interference from other cells while increasing desired signal power. CoMP includes uplink CoMP in which a plurality of eNodeBs receives data transmitted from a UE in coordination, and downlink CoMP in which a plurality of eNodeBs transmits data to one UE in coordination. Both are important technologies. Moreover, if CoMP is applied to the uplink, low transmit power will do. Accordingly, a UE (among others, a MTC device) is given the effect of reducing power consumption but has a problem in uplink coverage since the transmit power is restricted.
Regarding Decision on CoMP Sets
A combination of eNodeBs that perform CoMP for a certain UE, that is, constitute a coordinating group is hereinafter referred to as the Set of CoMP transmission point, or abbreviated to CoMP set.
In order to decide on the CoMP set, it is necessary to judge which eNodeBs are used to be effective for a UE. This is referred to as decision on the CoMP set, or point selection.
A method in which a UE receives reference signals from base stations, measures RSRP (Reference Signal Received Power) on a base station basis, and selects eNodeBs located at higher ranks as the CoMP set is cited as one of methods for point selection.
Proposed is, for example, a coordinating group setting method in which a base station transmits, to a user terminal, coordinating group setting signaling to which cell IDs of cells in a coordinating group selected for the user terminal are attached, and the user terminal sets the coordinating group with the cell IDs of the cells in the coordinating group selected for the user terminal, the cell IDs having been attached to the coordinating group setting signaling (see, for example, Patent Document 1).
Moreover, proposed is a radio communication method in which a base station allocates parts of the entire frequency band respectively to bands used for single base station transmission (first transmission mode) and multiple base station transmission (second transmission mode), and decides which transmission mode is used based on received feedback information on reception quality of one of the bands, in other words, restricts the feedback information to information on the reception quality of a part of the entire frequency band to reduce the amount of the feedback information (see, for example, Patent Document 2).
Moreover, proposed is a radio communication system in which each base station device receives, from a target terminal device, quality information on the quality of communication with base station devices capable of communicating with the target terminal device, and a base station device acquires schedule information on schedules of communication with terminal devices existing in cells of other base station devices, and selects, based on the quality information and schedule information, some base station devices to cause to function as base stations for the target terminal device (see, for example, Patent Document 3).
Request of Frequency of Point Selection
Point selection is required to be performed at fixed time intervals (in other words, the CoMP set is required to be updated at regular time intervals). In other words, there is a problem that the CoMP set of a UE is updated in a semi-static (Semi-Static) or dynamic (Dynamic) manner. Considering a temporal change in communication environment such as the movement of the UE, dynamic point selection to dynamically update the CoMP set is desired.
Basic Matters to be Considered in Dynamic Point Selection
If dynamic point selection is performed, it is necessary to consider a reduction in throughput due to the occupation of a reference signal in a communication sequence in the system, in other words, downlink overhead due to the reference signal.
Moreover, it is necessary to consider an increase in power consumption of the UE due to the UE's frequent measurements of the reference signal, and the like. Above all, if the UE is a MTC device, there is a request to reduce the cost of manual battery change and accordingly extremely low power consumption is required (which is described later).
Furthermore, it is also necessary to consider an increase in uplink overhead due to the feedback of the measurement result of the reference signal from the UE to the eNodeB.
In downlink CoMP, it is desired to perform dynamic point blanking. The reason is to avoid a reduction in the throughput of the entire system due to an increase in interference by an unnecessary eNodeB emitting unnecessary radio waves.
On the other hand, in dynamic point blanking in uplink CoMP, there is surely no problem in unnecessary emission since it is about reception on the eNodeB side. However, there is a problem that it is necessary to keep a resource block empty for another UE that is attempting to perform uplink transmission targeting the resource block, and unnecessary reception on the eNodeB side is not desired. The problem of the resource block appears as a phenomenon of a reduction in the throughput of the entire system in the end. Therefore, it is also desired in uplink CoMP to reduce the eNodeB's unnecessary reception by performing dynamic point blanking.